This invention relates to electrochromic display cells and concerns novel control means for an electrochromic display designed to improve the appearance and readability of the symbols displayed. The invention is particularly useful in the horological industry as a means of displaying the time in digital or analogue form.
An electrochromic display cell is constituted by an electrochromic material, capable of assuming two different appearances under the effect of electric currents and arranged in the form of separate electrode segments, separated from a common counter-electrode by an electrolyte. Associated electrical circuits allow a predetermined voltage to be applied to each segment individually with respect to the common counter-electrode. The current which then passes makes each of the segments take on one or other of two different appearances. These appearances are generally distinguished as the coloured, or displayed, state and the colourless or effaced state. The segments are arranged on the surface of the display cell so as to represent figures or other symbols, according to the combinations of coloured or colourless states they are capable of assuming. In displays of hours or dates, for example, it is customary to arrange seven segments for one character, so that this character can appear whenever required as any one of the numbers 0 to 9, according to the number and position of the segments made visible by colouring.
In some instances, several counter-electrodes are provided, each corresponding to one part of the electrode segments.
It is also known that one of the particular characteristics of electrochromic cells which, moreover, is a reason for their success in apparatuses supplied by low-powered batteries, e.g., watches, is the persistence of both states, the main function of the energy consumed being to effect changes in state, by colouring or effacing, and not to hold the cells in a predetermined state.
This type of behavior, which can be compared with that of capacitors, involves requirements specific to the supplying and control of the electrochromic display cells. The contract between a segment and the background of the display is determined by the electric charge injected per unit of surface of the segment to cause its change of state, the light absorption coefficient of the electrochromic material being proportional to this charge. It is essential for the quantity of charge injected at each change of state to be constant, precisely regulated and adapted to each segment. It is even more desirable that, in a display system like that for watches, where the characters change at a regular rate to represent in succession all the symbols, in a constantly repeated cycle, provision be made to limit the changes of state to the single segments which have to appear or disappear during the change from one figure to the next. Consequently, after each change of state, a segment must present a well determined contrast so that the displayed number is uniform. One also seeks to avoid variations occurring in the internal impedance of the display cells as a function of age and temperature, as well as those of differences in resistance of the connections connecting the segments to their control circuits. Finally, it is desirable to vary, from one segment to the other, the quantity of charge injected, at each change of state, to allow for differences in area between the segments.
The control circuits usually associated with electrochromic cells do not allow these objectives to be achieved. They are in fact simply constituted by electronic switches applying a constant voltage for a predetermined time to the segments to be controlled. The current which then circulates, and consequently the quantity of charge injected, is dependent upon various parameters which are difficult to regulate and vary from one segment to another. The contrast thus obtained is consequently not the same for all segments.
Swiss patent specification No. 609,408 describes an apparatus for controlling the segments of an electrochromic display which comprises for each segment a source of current capable of supplying a constant current for both directions of colour-change. This apparatus has the disadvantage of necessitating two sources of supply voltage supplying respectively a positive voltage and a negative voltage with respect to a mean potential. With such an apparatus, it is also very difficult, not to say impossible, to obtain, during the effacing stage, exactly the same quantity of charge as that supplied during the colouring stage.
If the colouring charge is larger than the effacing charge, even if only by a very small amount, the difference between these two charges accumulates in the segment, which then presents a residual colouring which increases with each change of state. The segment will consequently gradually become visible even when it should be effaced, and the contrast that it presents when it should normally be coloured increases, which can be bad for the cell in the long term.
In the opposite case, the voltage between the segment and the counter-electrode increases, when effacing finishes, to the value of the supply voltage, which also badly affects the display cell in the long term.
To obtain a contrast which is uniform and constant in time, the charges supplied to the segments during one of their changes of state must be individually matched by the charges applied during the other changes of state.